The invention relates to a circuit for driving a switching transistor which has a control terminal and a main current path, and during a switch-off phase (that is, in response to reversal of control terminal current to switch the transistor off) has a storage time delay between the control current reversal and the start of switching off of the main current path. The invention also relates to a method of driving such a switching transistor. The invention further relates to a line deflection circuit with such a driving circuit and a switched mode power supply with such a driving circuit.
DE-A-3518768 discloses a drive circuit for a switching transistor used in a switched mode power supply or a line deflection circuit. In a first period, the drive circuit supplies a base current to the switching transistor with a polarity such that the switching transistor conducts. In a second period, the drive circuit supplies a base current with an opposite polarity to switch off the switching transistor. It takes some time (usually referred to as the storage time) from the start of the second period before the accumulated charge in the switching transistor has been removed and the switching transistor starts switching off its main current path. With respect to the line deflection circuit wherein the switching transistor is a bipolar transistor, next, during the flyback period, a resonant circuit coupled to the collector of the switching transistor starts resonating thereby causing at the collector a flyback pulse with a half sinewave shape. During the first part of the flyback period, the voltage across the switching transistor main path increases while the current through the main path decreases rapidly to zero. The power dissipated in the switching transistor during this transition period wherein both a finite current and voltage occur, is further referred to as flyback losses. In DE-A-3518768 the base current of the switching transistor is controlled in response to the steepness of the flyback pulse to keep the flyback losses during the flyback period substantially constant. The dimensioning of the circuit is cumbersome because the base current has to be adjusted to a desired value in a situation wherein both the switching transistor has been selected with properties causing a maximal base current and the drive circuit has been dimensioned to cause a maximal base current. It is a further drawback of the prior art that extra components are required to determine the steepness of the flyback pulse.
It is, inter alia, an object of the invention to provide an optimal drive of a switching transistor without the need to know all the spreads of the drive circuit and the switching transistor.
To this end, a first aspect of the invention provides a circuit for driving a switching transistor. A second aspect of the invention provides a method of driving a switching transistor. A third aspect of the invention provides a line deflection circuit with such a driving circuit. A fourth aspect of the invention provides a switched mode power supply with such a driving circuit. Advantageous embodiments are defined in the dependent claims.
According to the first aspect of the invention, the storage time of the semiconductor switching element (the switching transistor) is measured to control the control signal supplied to the control terminal of the semiconductor switching element in a manner enabling the storage time to be kept substantially constant. The semiconductor switching element may be any suitable single switching element or a combination of switching elements. For example, the switching element is a bipolar transistor, a field effect transistor, a gate turn off device (GTO), or an insulated gate bipolar transistor (IGBT). Examples of the combination of switching elements are darlington arrangements, series or parallel arrangements. All these switching elements have in common that they show a delay (the storage time) between the instant that the control signal indicates, to the control electrode (the base or gate), that the switching element should be switched off and the instant the main path (collector-emitter, drain-source) starts switching off.
EP-A-0163302 discloses a driver circuit for a line deflection power transistor. The driver circuit controls the base current of the transistor in dependence on the current through the main path of the transistor to lower the dissipation in the transistor. To be compatible with existing phi2 controllers, it is an essential feature of this prior art that the base current is controlled in such a way that the storage time variation is inversely proportional to the end value of the collector current of the transistor.
In an embodiment of the invention in accordance with claim 3, the storage time is measured by using a counter which counts clock pulses from at the instant the switching transistor receives the command to switch off until the instant the main path starts switching off. This way of measuring the storage time is especially easy and cheap when the driver circuit comprises digital circuits or a microprocessor, which is becoming more and more the case in synchronization circuits in display apparatuses. In this case, a very flexible solution is available: during production, the desired storage time can be programmed into the synchronization circuit via a bus. The desired storage time may depend on the type of switching element used, or on the application of this switching element. DE-A-3518768 measures the steepness of the flyback pulse, which is more complicated and less flexible than measuring the storage time and controlling the storage time.
In an embodiment of the invention in accordance with claim 4, the storage time is controlled by varying the amount of current in the control electrode of the switching element.
In an embodiment of the invention in accordance with claim 5, the storage time is controlled by varying the duty cycle of the control signal supplied to the control electrode. It is also possible to vary the slope of the current in the control electrode to control the storage time.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.